Abstract

Introduction: The CBP501 calmodulin-binding peptide is an anti-cancer drug candidate that has completed two Phase II clinical trials for patients with malignant pleural mesothelioma and non-small cell lung carcinoma (NSCLC). CBP501 was previously described as a unique G2 checkpoint-directed agent and as an enhancer of cisplatin (CDDP) uptake. In a post-trial analysis of patients with NSCLC, it was found that overall survival (OS) was prolonged in a subpopulation of patients with normal white blood cell counts (WBC) (WBC < 8000). Here we show that modulation of immunogenic cell death might be a novel function of CBP501. Based on this novel activity of CBP501, combined treatment with PD-1 blocking antibodies in vivo was also explored.

Results: In vitro, CDDP in combination with CBP501 elicited increased death in CT26WT cells, as well as an increase in different indicators of immunogenic cell death. These indicators include induction of phospho-eIF2-alpha, increase in cell surface-exposed calreticulin, and extracellular release of HMGB1. In an in vivo BALB/c mouse model, CDDP-treated mice showed a reduction of tumor growth by 52.7% as compared to vehicle-treated mice. CBP501 + CDDP showed an additional reduction of tumor growth by 63.1% as compared to vehicle-treated mice. Treatment with anti-mPD-1 antibody alone showed a slight reduction of tumor growth by 25.2% as compared to vehicle-treated mice. However, combined treatment with anti-mPD-1 + CDDP or anti-mPD-1 + CDDP + CBP501 showed significant reductions in the tumor growth in comparison to the vehicle-treated mice by 69.3% and 78.7%, respectively.

Conclusion: These results suggest that the anti-tumor activity of combined CBP501 + CDDP treatment is potentiated by inducing immunogenic cell death. This novel effect might contribute to the prolonged OS found in the phase II clinical trials. Combined treatments that include the anti-mPD-1 immune-checkpoint inhibitor were effective and shall be examined further. Further, immunohistochemical analyses of the effects of this combined treatment on tumor microenvironment are under way.